A. Field of the Invention
The present invention relates to an optical filter made of inorganic material for blue light, provided for the application to a display device consisting of a combination of plural kinds of displaying elements emitting different colored lights respectively, particularly with high contrast based on antireflection with low reflectance of ambient light.
B. Description of the Prior Art
Generally, it is required for various kinds of display devices, for instance, a cathode ray tube, a gas-discharge display panel and a low-velocity electron-beam fluorescent display panel, to increase not only the brightness but also the contrast of the display by lowering the reflectance of the displaying surface. If the required level of brightness of the display can be obtained in any manner, the quality of display can be occasionally improved by increasing the contrast thereof.
For lowering the reflectance of the displaying surface in order to increase the contrast thereof, the following various measures have been conventionally adopted.
(i) A neutral density (ND) filter of absorbing type is sticked or combined to a surface of the display screen which is made of glass plate and the like.
(ii) The light absorption of the display screen at a region thereof, at which the radiation spectral intensity of fluorescent material forming the display element is low, is increased by adding a rare earth element, for instance, Nd.sub.2 O.sub.3 into a material, for instance, glass which is a component of the display screen.
(iii) The light absorption of the display screen is increased at the light wavelength region which is unavailable for the display radiation region by mixing a pigment into the fluorescent material forming the display element.
(iv) The absorption of ambient light is increased by sticking a black material on portions of the display element which consist of other than the fluorescent material.
(v) Granular pigments sticked on the fluorescent material are employed as for a filter.
However, the above conventional measures for increasing the contrast have the following defects respectively.
The increase of contrast according to the application of the ND filter as mentioned in Item (i) is accompanied by decreased brightness.
Although the result obtained by adding the rare earth element into glass as mentioned in Item (ii) is more favorable than that obtained by the above measure of Item (i), the luminous reflectance is not decreased, since the light transmittance of the added material in the vicinity of the wavelength 555 nm is high.
The mixing of the pigment into the fluorescent material as mentioned in Item (iii) is effective for the CRT display device which has a very bright display. However, the application thereof onto the other display devices which cannot afford to lose only brightness seems to be difficult, meanwhile the lowered degree of reflectance thereof seems to be no more than one half at most in practice.
The sticking of black absorbing material on other portions than that emitting the light as mentioned in Item (iv) is effective for all the above display devices. However, this measure is no more than subsidiary.
The sticking of granular pigments onto the fluorescent material as for the filter is not useful in practice, since the pigment contained in the filter presents the granulation, the boundary of which causes the increased reflection based on the scattering of light. In addition, a gap between the front glass and the filter layer, even if trifling, causes the reflection of light at the boundary thereof and hence the lowered contrast.
The present inventors have previously conceived a color display device employing an optical filter made of inorganic material which effectively increases the contrast of the display device, particularly, the color picture display device by removing the above described conventional defects and hence by suppressing the reduction of the brightness of display and have filed a patent application thereof referred to Japanese Patent Laid-open Publication No. 59-36,280.
As described in this publication, in the display device formed of a combination of plural kinds of respectively different displaying elements, the contrast of the picture consisting of colored lights emitted by those displaying elements respectively is increased by almost four times by employing an optical filter formed of substantially transparent inorganic glasses through which those colored lights are transmitted respectively, in comparison with the contrast obtained without this optical filter, that is, the combination of three colored filters of red, green and blue. Further, the reduction of the brightness, which accompanies the application of the filters, is about 20%.